There is a need to improve the performance and lower the cost of electrochemical membrane sensors, such as dissolved oxygen sensors of the galvanic or polargraphic types, which employ a membrane as an isolation barrier. These sensors are typically used as immersible transducers for the measurement of the quality of test liquid, such as industrial and domestic water and wastewater. They employ a membrane covered electrode system to minimise electrode poisoning or other interferences which would otherwise be experienced by the action of impurities in the test liquid.
In general, the membrane sensors described herein employ a molecular detection system consisting of two solid metal electrodes in contact with a special electrolyte which are separated from the test liquid by a selectively permeable membrane. The basic difference between the galvanic and the polargraphic systems is that, in the former, the electrode reaction is spontaneous (similar to that in a fuel cell), while in the latter, an external source of applied voltage is needed to polarise the indicator electrode.
In the current state of the art, the polargraphic dissolved oxygen sensor (Clark Cell) typically consists of a gold cathode and a silver anode surrounded by a KCl electrolyte. Polyethylene or flourocarbon membranes are commonly used because of their high oxygen permeability and mechanical strength. The Galvanic dissolved oxygen sensor (Mackereth Cell) typically employs gold or silver in the cathode with a lead or cadmium anode. In both types of Cell, the diffusion of oxygen molecules through the membrane causes an electrical current to flow through the Cell which is linearly proportional to the concentration of dissolved oxygen (DO) in the test liquid.
Common DO sensors of this type require stirring or movement of the test liquid to ensure that the test liquid nearest to the membrane does not become depleted of dissolved oxygen and so cause a slower than normal response. When used in dirty liquids, such as heavily laden river water or sewage, these common DO sensors are easily desensitised by deposits of chemical, bacterial and biological contaminants, thus requiring regular membrane replacement.
The present invention overcomes the need for stirring the fluid under test and includes a self cleaning feature to enable more reliable operation in dirty fluids.